diff --git a/markdown_it/rules_block/html_block.py b/markdown_it/rules_block/html_block.py
index 3a130907..1ff23d76 100644
--- a/markdown_it/rules_block/html_block.py
+++ b/markdown_it/rules_block/html_block.py
@@ -14,16 +14,18 @@
 # last argument defines whether it can terminate a paragraph or not
 HTML_SEQUENCES: list[tuple[re.Pattern[str], re.Pattern[str], bool]] = [
     (
-        re.compile(r"^<(script|pre|style|textarea)(?=(\s|>|$))", re.IGNORECASE),
-        re.compile(r"<\/(script|pre|style|textarea)>", re.IGNORECASE),
+        re.compile(r"^<(?a:(script|pre|style|textarea))(?=(\s|>|$))", re.IGNORECASE),
+        re.compile(r"<\/(?a:(script|pre|style|textarea))>", re.IGNORECASE),
         True,
     ),
     (re.compile(r"^<!--"), re.compile(r"-->"), True),
     (re.compile(r"^<\?"), re.compile(r"\?>"), True),
-    (re.compile(r"^<![A-Z]"), re.compile(r">"), True),
+    (re.compile(r"^<![A-Za-z]"), re.compile(r">"), True),
     (re.compile(r"^<!\[CDATA\["), re.compile(r"\]\]>"), True),
     (
-        re.compile("^</?(" + "|".join(block_names) + ")(?=(\\s|/?>|$))", re.IGNORECASE),
+        re.compile(
+            "^</?(?a:(" + "|".join(block_names) + "))(?=(\\s|/?>|$))", re.IGNORECASE
+        ),
         re.compile(r"^$"),
         True,
     ),
diff --git a/tests/test_port/fixtures/commonmark_extras.md b/tests/test_port/fixtures/commonmark_extras.md
index 0f835ae3..98ce1ad5 100644
--- a/tests/test_port/fixtures/commonmark_extras.md
+++ b/tests/test_port/fixtures/commonmark_extras.md
@@ -871,3 +871,12 @@ b
 <p>c</p>
 </blockquote>
 .
+
+HTML block type 4 (declaration) starts with `<!` plus any ASCII letter, not only uppercase (CommonMark 4.6, start condition 4). A lowercase declaration must interrupt a paragraph.
+.
+foo
+<!bar baz>
+.
+<p>foo</p>
+<!bar baz>
+.
diff --git a/tests/test_port/fixtures/issue-fixes.md b/tests/test_port/fixtures/issue-fixes.md
index 596c6cd3..fefef099 100644
--- a/tests/test_port/fixtures/issue-fixes.md
+++ b/tests/test_port/fixtures/issue-fixes.md
@@ -140,3 +140,107 @@ foo
 <p>foo
 --&gt;</p>
 .
+
+#437 a lowercase declaration interrupts a paragraph, as an uppercase one does
+.
+para
+<!doctype html>
+
+para
+<!DOCTYPE html>
+.
+<p>para</p>
+<!doctype html>
+<p>para</p>
+<!DOCTYPE html>
+.
+
+#437 Unicode lookalikes that case-fold to ASCII (U+017F, U+0130, U+0131) do not start an HTML block
+.
+para
+<ſcript>
+text
+
+para
+<ſtyle>
+text
+
+para
+<dİv>
+text
+
+para
+<dıv>
+text
+.
+<p>para
+&lt;ſcript&gt;
+text</p>
+<p>para
+&lt;ſtyle&gt;
+text</p>
+<p>para
+&lt;dİv&gt;
+text</p>
+<p>para
+&lt;dıv&gt;
+text</p>
+.
+
+#437 a Unicode lookalike does not close a type 1 HTML block
+.
+<script>
+x
+</ſcript>
+after
+
+more
+.
+<script>
+x
+</ſcript>
+after
+
+more
+.
+
+#437 an ASCII mixed-case tag name still starts and closes an HTML block
+.
+para
+<ScRiPt>
+text
+</sCrIpT>
+after
+.
+<p>para</p>
+<ScRiPt>
+text
+</sCrIpT>
+<p>after</p>
+.
+
+#437 a no-break space (U+00A0) after a type 6 tag name still starts an HTML block
+.
+para
+<div class="x">
+text
+.
+<p>para</p>
+<div class="x">
+text
+.
+
+#437 a no-break space (U+00A0) after a type 1 tag name still starts an HTML block
+.
+para
+<pre >
+
+*a*
+</pre>
+.
+<p>para</p>
+<pre >
+
+*a*
+</pre>
+.